fix: keep RSA-OAEP-256 digest correct in runtimes that ignore oaepHash

lib/mcapi/crypto/jwe-crypto.js

@@ -26,6 +26,12 @@ function JweCrypto(config) {
 
   this.encryptedValueFieldName = config.encryptedValueFieldName;
 
+  // Only parsed if this runtime needs the node-forge fallback, so callers on
+  // node never pay for it. A config may also legitimately omit the private
+  // key.
+  let parsedPublicKey = null;
+  let parsedPrivateKey = null;
+
   /**
    * Perform data encryption
    *
@@ -52,14 +58,31 @@ function JweCrypto(config) {
     const secretKey = nodeCrypto.randomBytes(32);
     const secretKeyBuffer = Buffer.from(secretKey, c.BINARY);
 
-    const encryptedSecretKey = nodeCrypto.publicEncrypt(
-      {
-        key: this.encryptionCertificate,
-        padding: nodeCrypto.constants.RSA_PKCS1_OAEP_PADDING,
-        oaepHash: "sha256",
-      },
-      secretKeyBuffer
-    );
+    let encryptedSecretKey;
+    if (honoursOaepHash(this.encryptionCertificate)) {
+      encryptedSecretKey = nodeCrypto.publicEncrypt(
+        {
+          key: this.encryptionCertificate,
+          padding: nodeCrypto.constants.RSA_PKCS1_OAEP_PADDING,
+          oaepHash: "sha256",
+        },
+        secretKeyBuffer
+      );
+    } else {
+      if (!parsedPublicKey) {
+        parsedPublicKey = forge.pki.certificateFromPem(
+          this.encryptionCertificate
+        ).publicKey;
+      }
+      encryptedSecretKey = Buffer.from(
+        parsedPublicKey.encrypt(
+          secretKeyBuffer.toString(c.BINARY),
+          "RSA-OAEP",
+          createOAEPOptions()
+        ),
+        c.BINARY
+      );
+    }
 
     const iv = nodeCrypto.randomBytes(16);
 
@@ -110,14 +133,29 @@ function JweCrypto(config) {
     const encryptedText = Buffer.from(jweTokenParts[3], c.BASE64);
     const authTag = Buffer.from(jweTokenParts[4], c.BASE64);
 
-    let secretKey = nodeCrypto.privateDecrypt(
-      {
-        key: this.privateKey,
-        padding: nodeCrypto.constants.RSA_PKCS1_OAEP_PADDING,
-        oaepHash: "sha256",
-      },
-      Buffer.from(encryptedSecretKey, c.BINARY)
-    );
+    let secretKey;
+    if (honoursOaepHash(this.encryptionCertificate)) {
+      secretKey = nodeCrypto.privateDecrypt(
+        {
+          key: this.privateKey,
+          padding: nodeCrypto.constants.RSA_PKCS1_OAEP_PADDING,
+          oaepHash: "sha256",
+        },
+        Buffer.from(encryptedSecretKey, c.BINARY)
+      );
+    } else {
+      if (!parsedPrivateKey) {
+        parsedPrivateKey = forge.pki.privateKeyFromPem(this.privateKey);
+      }
+      secretKey = Buffer.from(
+        parsedPrivateKey.decrypt(
+          Buffer.from(encryptedSecretKey, c.BINARY).toString(c.BINARY),
+          "RSA-OAEP",
+          createOAEPOptions()
+        ),
+        c.BINARY
+      );
+    }
 
     let decryptionEncoding = JSON.parse(jweHeader).enc;
     let gcmMode = true;
@@ -267,6 +305,89 @@ function validateFingerprint(config, contains) {
   }
 }
 
+/**
+ * Does this runtime's crypto honour the `oaepHash` option?
+ *
+ * node validates the digest name and throws on an unrecognised one. The
+ * crypto-browserify polyfill ignores `oaepHash` altogether and hard-codes
+ * SHA-1, so it accepts a nonsense value without complaint. Attempting a wrap
+ * under a bogus digest is therefore a cheap and side-effect free probe.
+ *
+ * This matters because a polyfilled `publicEncrypt` reports no error while
+ * producing a token whose header advertises RSA-OAEP-256 over a key that was
+ * actually wrapped with SHA-1. The failure only surfaces at the recipient.
+ *
+ * node parses the key before it resolves the digest, so the probe needs a
+ * usable key to reach the digest check at all. It takes the encryption
+ * certificate, which config validation requires, for both directions; a
+ * decrypt-only caller still has one.
+ *
+ * The native path is chosen only on positive evidence: node's own
+ * invalid-digest error. Any other failure, such as a key the runtime cannot
+ * parse, says nothing about the runtime. It is not memoised and that call
+ * takes the node-forge path, which applies SHA-256 regardless.
+ *
+ * Memoised once the answer is known: it is a property of the bundle, not of
+ * the key.
+ *
+ * @private
+ */
+let nativeOaepHash = null;
+function honoursOaepHash(certificatePem) {
+  if (nativeOaepHash !== null) {
+    return nativeOaepHash;
+  }
+  if (
+    typeof nodeCrypto.publicEncrypt !== "function" ||
+    !nodeCrypto.constants
+  ) {
+    // `resolve.fallback.crypto = false`, as set in this repo's webpack config,
+    // leaves an empty module behind. Treat anything unusable as unsupported.
+    nativeOaepHash = false;
+    return nativeOaepHash;
+  }
+  try {
+    nodeCrypto.publicEncrypt(
+      {
+        key: certificatePem,
+        padding: nodeCrypto.constants.RSA_PKCS1_OAEP_PADDING,
+        oaepHash: "mcapi-oaep-support-probe",
+      },
+      Buffer.alloc(1)
+    );
+    // Accepted a digest that does not exist, so it is not using the option.
+    nativeOaepHash = false;
+  } catch (err) {
+    if (!err || err.code !== "ERR_OSSL_EVP_INVALID_DIGEST") {
+      return false;
+    }
+    nativeOaepHash = true;
+  }
+  return nativeOaepHash;
+}
+
+/**
+ * Build node-forge OAEP options for RSA-OAEP-256.
+ *
+ * The JWE header emitted by this module is always `"alg": "RSA-OAEP-256"`, so
+ * the label digest and the MGF1 digest are both SHA-256. MGF1 has to be set
+ * explicitly: node-forge defaults it to SHA-1, which yields a key wrap that no
+ * conforming RSA-OAEP-256 implementation can undo.
+ *
+ * Mirrors createOAEPOptions in field-level-crypto.js.
+ *
+ * @private
+ */
+function createOAEPOptions() {
+  const md = forge.md.sha256.create();
+  return {
+    md: md,
+    mgf1: {
+      md: md,
+    },
+  };
+}
+
 /**
  * @private
  */